Your SX80 must be on a public IP address, external entities are trying to use your system to perform PTSN toll fraud calls. You could turn off the SIP ListenPort on the SX80 as Balazs suggests, but that will only work if they are using SIP. However, lately we've been seeing this issue coming in on the H323 ports, and the only way to prevent them is to block all incoming traffic to the endpoint, and only allow specific known IPs inside the firewall, or use NAT.
This has been asked many time in the forums, here is one discussions with several links to others that contain workarounds.
cisco-vcs-ghost-or-fake-uris-dialing-attempts
